Electrical resistance alloy and method of producing same

ABSTRACT

AN ELECTRICAL RESISTANCE ALLOY CHARACTERIZED BY A TEMPERATURE COEFFICIENT OF RESISTANCE WHICH MAINTAINS AN ESSENTIALLY CONSTANT FIXED VALUE APPROACHING ZERO OVER A SELECTED TEMPERATURE RANGE AND COMPRISING, AS EXPRESSED IN PERCENT BY WEIGHT, THE FOLLOWING MAJOR CONSTITUENTS:   PERCENT ALUMINUM 2.75-3.75 MANGANESE 4.0-5.5 SILICON 0.70-1.25 CHROMIUM 17.5-22.5 NICKEL BALANCE   TO THIS IS ADDED AS A MINOR CONSITUENT AT LEAST ONE DEOXIDIZER SELECTED FROM THE CLASS CONSISTING OF ZIRCONIUM CALCIUM AND MAGNESIUM IN AN AMOUNT UP TO 0.1%. THE ALLOY IS PRODUCED BY A PROCESS IN WHICH IT IS STRANDED ANNEALED, RAPID COOLED, HEAT TREATED AND SLOW COOLED.

United States Patent Ofice 3,598,578 Patented Aug. 10, 1971 US. Cl. 75-171 Claims ABSTRACT OF THE DISCLOSURE An electrical resistance alloy characterized by a temperature coefiicient of resistance which maintains an essentially constant fixed value approaching zero over a selected temperature range and comprising, as expressed in percent by weight, the following major constituents:

Percent Aluminum 2.75-3.75 Manganese 4.0-5.5 Silicon 0.70-1.25 Chromium 17.5-22.5 Nickel Balance To this is added as a minor constituent at least one deoxidizer selected from the class consisting of zirconium, calcium and magnesium in an amount up to 0.1%. The alloy is produced by a process in which it is strand annealed, rapid cooled, heat treated and slow cooled.

BACKGROUND OF THE INVENTION The temperature coefficient of resistance of a material is defined over any temperature range as follows:

where TCR is the temperature coefficient of resistance between temperature T and T where T T R is the resistance at temperature T R is the resistance at temperature T and R is the resistance at a reference temperature, which is usually either 20 or 25 C.

An ideal resistance alloy would, among other parameters, exhibit a constant value of resistance over the entire range of temperatures to which the alloy is to be subjected. Since no alloy is ideal, the objective is to obtain an alloy that approaches one. Therefore, a TCR that approaches zero in the temperature ranges from 55 C. to +25 C. to 105 C., the normal ranges of interest, will also have a low incremental TCR between these two ranges. This can approximate one part per million per degree C. (p.p.m./). However, commercially available alloys exhibit non zero values of TCR over each of these ranges.

Certain known alloys have exhibited incremental TCR values of one p.p.m./ C. For example, US. Pat. 3,406,058 discloses an electrical resistance alloy having a resistivity of 860 ohms per circular mill foot and an incremental TCR of about one p.p.m./ C. over a temperature range of 55 C. to 125 C. The alloy composition, as expressed in percent by weight, is l5%-25% chromium, 4.5%-5.5% manganese, 4%-4.5% aluminum, 0.90%-l.25% silicon, balance nickel.

This known alloy while useful for a variety of applications, has two inherent disadvantages. First, the alloy, during processing, must be formed into wire and heated rapidly and uniformly by strand annealing in order to maintain the incremental TCR at one p.p.m./ C. This process can be applied successfully only to wires of small diameter as for example .0025 inch or less. When the diameter of the wire is increased appreciably beyond this value, the incremental TCR increases to unacceptable values. Secondly, the alloy is difficult to forge and draw.

I have discovered that both the above disadvantages can be overcome by producing an alloy of the same general type with a substantially reduced aluminum content. More particularly, my alloy is heated slowly whereby wire of any diameter up to about .010 inch can be brought to temperature and cooled at the same rate While the incremental TCR is held at one p.p.m./ C. Moreover, my alloy can be forged and worked more easily than an alloy with higher aluminum content.

SUMMARY OF THE INVENTION My alloy has the following major constituents, as expressed in percent by weight:

Percent Aluminum 2.75-3.75 Manganese 4.0-5.5 Silicon 0.70-1.25 Chromium l7.5-22.5 Nickel Balance To this is added up to 0.1% of at least one deoxidizer selected from the group consisting of zirconium, calcium and magnesium.

My alloy exhibits an incremental TCR of one p.p.m./ C. or less over the temperature range of interest, regardless of the diameter of the Wire in which it is formed. My alloy can be forged and worked without difiiculty.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS A typical alloy in accordance with my invention had the following composition:

This alloy was cast, hot rolled, and subsequently drawn to wire and annealed at a selected temperature within the range 1700 F. to 2000 F. as for example 2000 F. for a few seconds and then was rapidly quenched. In this condition, the resistivity of the wire was 767 ohms per circular mil foot. The TCR cold, ie, over the range 55 to 25 C., was +52 p.p.m./ C., and the TC-R hot, be tween +25 to C., was +49 parts p.p.m./ C.

After a subsequent heat treatment for about one hour at 940 F. in a hydrogen atmosphere with the wire wound upon a spool, the resistivity of my alloy increased to about 830 ohms per circular mil foot. Typically, the-TCR cold was 0 p.p.m./ C. while the TCR hot was 1 p.p.m./ C.

Unlike strand annealing techniques, this heat treatment is effective regardless of the diameter of the wire being treated. For example, the same results can be obtained for diameters ranging between .0004 inch and .010 inch.

By varying the temperature of the second heating step within the range 900 F. to 1000 F. and by varying the time within the range of approximately thirty minutes to ninety minutes, the TCR hot and the TCR cold values can be varied, while maintaining the overall value of incremental TCR at a value of one p.p.m./ C.

While I have described my invention with respect to preferred embodiments, my invention is to be limited only by the terms of the claims which follow.

Percent Aluminum 2.75-3.75 Manganese 4.0-5.5 Silicon 0.70-1.25 Chromium 17.5-22.5 Nickel Balance 2. An alloy as set forth in claim 1 further including up to 0.1% of at least one deoxidizer selected from the group consisting of zirconium, calcium and magnesium.

3. A method for producing an electrical resistance alloy with a resistivity of at least 800 ohms per circular mil foot and a temperature coefficient of resistance which does not vary more than one part per million over the temperature range from 55 C. to 25 C. and from 25 C. to 105 C. from a starting alloy comprising, as expressed in percent by weight, 2.75%3.75% aluminum, 4.0%- 5.5% manganese, 0.70%1.25% silicon, 17.5 %22.5 chromium, balance nickel, which comprises the steps of:

(a) drawing said starting alloy to a wire of desired dimension;

(b) strand annealing said wire to a temperature between 1700 F.2000 F. for a short period;

(c) rapid cooling said annealed wire;

(d) heat treating said cooled wire at a temperature of 900 F.1000 F. in a reducing atmosphere for a longer period; and

(e) slow cooling said treated wire to produce said electrical resistance alloy.

4. A method as set forth in claim 3 wherein step (c) is carried out by quenching.

5. A method as set forth in claim 3 wherein step (b) uses a period of several seconds and step (d) uses a period of at least thirty minutes.

References Cited UNITED STATES PATENTS 2,533,736 12/1950 Lohr 75171 3,406,058 10/1968 Poch et al 75-171 RICHARD O. DEAN, Primary Examiner US. Cl. X.R. 148-115, 13, 20.3 

